Fault sensing and recovery may be conducted at different levels in various types of optical transmission networks to avoid network failures and to assure network performance. For example, an element management system (EMS) may be used to supervise, manage and detect faults in network elements of a network. A network management system (NMS) may be used to supervise and manage the overall optical network by communicating with several EMSs. Based on information derived from peer EMSs, any EMS may initiate fault recovery without requiring centralized NMS participation.
In an optical communication system, for example, terminal or cable stations may be interconnected by cable segments to form a network. The network elements in an optical communication system may include equipment located at a cable station (e.g., terminal equipment and power feed equipment) as well as equipment connected to the cable station (e.g., repeaters and equalizers). In such a system, an EMS may be located at a cable station (or at a separate location) and used to manage the network elements associated with this cable station. An NMS may be located at one of the cable stations or at a separate location for managing the overall optical communication system and performing fault recovery based on communication with the EMSs.
Undersea optical fiber communication systems may include a relatively long trunk segment that may be terminated at a transmitting and/or receiving trunk terminal or cable station. The optical fiber transmission system may further include one or more optical devices, e.g., branching units and/or repeaters, situated along its trunk. Each branching unit (BU) may be connected to a branch segment that terminates in a transmitting and/or receiving branch terminal or cable station. Each branching unit may include an optical add/drop multiplexer (OADM) configured to add and/or drop data signals at the branching unit. Each trunk and/or branch terminal may be on or near dry land. The relatively long trunk system may run underwater, e.g., along an ocean floor. The optical devices may also be positioned on the ocean floor at a relatively long distance, e.g., fifty kilometers or more, from dry land.
Each trunk segment and/or branch segment may include one or more optical fibers. Each optical fiber may be configured to carry a plurality of multiplexed optical channels using, e.g., wavelength division multiplexing. An optical fiber may be susceptible to a fault, e.g., a cut or damaged fiber that may degrade or prevent propagation of one or more signals. The degradation and/or loss of signal may be detected at a receiving branch or trunk terminal.
Optical fiber communication systems that include OADMs may present special challenges for fault recovery in the event of a trunk segment or branch segment failure. There may be significant interdependence between trunk and branch traffic. For example, in some situations, trunk traffic may be impaired as a result of a branch fault. In another example, a trunk fault may not affect signals transmitted from a surviving portion of the trunk to a branch, but signals from the branch to the surviving portion of the trunk may be adversely affected. Whether traffic is affected and to what degree depends on several factors including OADM architecture, lengths of surviving fiber segments, location and type of fault, relative distances between branching units and trunk terminals and/or powering configuration of each branching unit.
Generally, alternate routes that include additional optical fibers may not be available for rerouting a signal from a faulty fiber segment, e.g., protection switching. This may be especially true of undersea and/or remote terrestrial optical communication systems where laying optical fibers may be difficult and expensive. Additionally or alternatively, for reliability considerations in fault detection and communication, it may be desirable that fault recovery not be reliant on a single centralized network management system but rather be distributed over the system. Accordingly, there is a need for distributed fault sensing and recovery in a communication system.
Fault recovery in a OADM network involves limiting the impairments experienced by the portions of the network not directly affected by the fault because the resulting optical network signal characteristics are no longer compatible with the planned optical amplification scheme. During recovery, optical signal sources are managed to recreate, as much as possible, the original optical network signal loading.